The hot press molding is widely known, in which the workpiece is heated to the predetermined temperature and put in to the mold, and the workpiece is kept in contact with the mold, thereby quenching and molding the workpiece into the desired shape.
The molding accuracy of the hot press molding (dimensional accuracy, quenchability and the like) is determined by the cooling gradient while quenching (cooling speed). Thus, measuring the temperature of the hot press mold and the workpiece in the mold and controlling the temperature are required as an important technique.
For example, JP 2005-199300 A discloses a temperature sensor which is disposed at the surface of the mold and measures the temperature of the workpiece, and a technique of controlling the motion speed of the pressing apparatus which actuates the mold on the basis of the measured temperature.
Due to the technique of JP 2005-199300 A, the temperature data detected by the temperature sensor allows control of the temperature of the workpiece under hot press molding. However, there are various problems with arrangement of the temperature sensor.
(1) If the temperature sensor is a contact sensor, the temperature sensor is arranged projecting outward from the molding surface. In such case, the surface accuracy is degraded because of the projection and therefore the molding accuracy goes down. Moreover, the temperature sensor is easily broken by the impact of the pressure. (2) If the temperature sensor is a non-contact sensor, it is difficult to accurately measure the rapid change in temperature of the workpiece (rapid cooling in quenching), and if the non-contact sensor is a precision measuring equipment such as an infrared radiation thermometer, there is a problem with the durability against impact pressure.
As mentioned above, the conventional technique did not solve the practical problems about the measurement of the temperature of the workpiece while hot press molding. Therefore, the quality of the entire molded product is not guaranteed.
Further, in the conventional hot press molding, the workpieces are continuously put in and the hot press molding is continuously operated. The quenching process causes the increase of the temperature of the mold. When the temperature of the mold is increased above the cooling capability thereof, the mold would not be cooled down, so that the cooling speed for quenching (the enough temperature difference between the mold and the workpiece) is not satisfied and the quenching defect may occur. In the case where the quenching defect occurs, it is difficult to determine when the defect occurs, and thereby all of the molded products made by the same processes should be checked. Accordingly, there is a practical problem remained.